1. Field of the Invention
An aspect of this disclosure relates to an optical scanning device and an optical scanning unit.
2. Description of the Related Art
A fixed (immobile) mirror used, for example, for a projector with a light source emitting a laser beam in a visible light range of 400-700 nm is typically produced by forming a metal film on a substrate of, for example, quartz or glass and forming a dielectric multilayer film composed of a low-refractive index film and a high-refractive index film on the metal film, to achieve a high reflectance. Japanese Laid-Open Patent Publication No. 2006-220903, for example, discloses a mirror including a substrate and a multilayer film composed of nine or more layers. The multilayer film includes a metal film formed on the substrate, and a dielectric multilayer film formed by stacking a low-refractive index film and a high-refractive index film alternately on the metal film.
Here, because the incident angle of incident light on such a fixed mirror is constant, the incident angle dependence of reflectance is not considered in determining the film configuration of the fixed mirror.
On the other hand, a mirror for an optical scanning device is used at various incident angles (e.g., about 0 through 50 degrees) in relation to the position of a light source, and the mirror itself oscillates (e.g., about ±10 degrees). Therefore, the film configuration of a mirror for an optical scanning device needs to be determined such that a high reflectance greater than a predetermined value can be achieved even when the incident angle changes.
Also, because a relatively thick (e.g., several mm) substrate can be used for a fixed mirror, the substrate is hardly deformed even when the total thickness of films formed on the substrate is large. That is, even when the number of stacked films is increased and the total thickness of the films increases, it does not cause a great problem. Accordingly, for a fixed mirror, no research is being made to reduce the number of stacked films and thereby reduce the total thickness.
On the other hand, because a mirror for an optical scanning device oscillates, a relatively thin (e.g., about several tens to several hundred μm) substrate needs to be used for the mirror. When the number of films stacked on a relatively thin substrate increases and the total thickness of the films increases, the substrate may be deformed. Therefore, in the case of a mirror for an optical scanning device, it is preferable to reduce the number of films stacked on a substrate and thereby reduce the total thickness of the films.